I am using SIMPSON to simulate DQ signals generated by the pulse sequence SPC5. The input file of my simulation is modified from the input file of Post-C7, which is written in the original SIMPSON paper (Bak, M.; Rasmussen, J. T.; Nielsen, N. C. Journal of Magnetic Resonance 2011, 213 (2), 366–400.) My input file is as followed:

#Cb, Ca, C' in order

#parameters from Journal of Magnetic Resonance 164 (2003) 270–285 (for alanine)

spinsys {

channels 13C

nuclei 13C 13C 13C

dipole 1 2 -2147 0 69 111

dipole 2 3 -2117 0 0 0

dipole 1 3 -475 0 34 111

}

par {

spin_rate 25000

sw spin_rate/2.0

np 64

crystal_file rep100

gamma_angles 40

start_operator Inz

detect_operator -Inz

verbose 1101

}

proc pulseq {} {

global par

maxdt 1

#matrix set detect totalcoherence {2}

set rf [expr 5.0*$par(spin_rate)]

set t90 [expr 0.25e6/$rf]

set t270 [expr 0.75e6/$rf]

set t360 [expr 1e6/$rf]

reset

for {set i 0} {$i < 5} {incr i} {

set ph [expr $i*360.0/5.0]

pulse $t90 $rf $ph

pulse $t360 $rf [expr $ph+180]

pulse $t270 $rf $ph

}

store 1

acq $par(np) 1

}

proc main {} {

global par

fsave [fsimpson] $par(name).fid

}

I used "-Inz" first for detect_operator as in the original paper and got a build-up curve with intensity ~0.75 (as attached). But I want to obtain DQ signals so I change the detect_operator to be "–(I1p*I2p+I1m*I2m)", which stands for DQx. However I got a build-up curve with intensity ~0.0007, too low for the DQ conversion efficiency (as attached). I also tried "matrix set detect totalcoherence {2}" in the proc pulseq part to obtain DQ signals but had the intensity with similar intensity.

Does anyone know what’s wrong with my coding? Thanks in advance.